The invention pertains to the field of CCD video probes, and in particular to long borescopes or similar medical devices that have long cables coupling the drive electronics assembly to the imager assembly.
CCD (charge coupled device) imagers require the use of a reset-gate pulse to discard charge from previous pixels before each succeeding pixel is read out from the imager. This signal is both attenuated and distorted as it passes through a coaxial cable, with the effect being stronger for long, small diameter cables such as used in borescopes. The distortion of the reset-gate pulse causes it to spread out in time. This spreading and reduced amplitude cause both fixed-pattern noise and random noise to appear in the picture. This results in a maximum usable length for a given imager/cable/image processing electronics system.
Non-linear amplifiers located near the imager can be used to regenerate the original waveform for the reset-gate. However, this approach does not eliminate all of the noise generating mechanisms, and incurs size penalties at the imager head. This is unacceptable for medical and other devices which must be kept small.
Briefly stated, picture noise in a CCD video probe is reduced by producing a faithful reproduction of a reset-gate signal received from a timing generator to an imager by using a cable compensator network. The network contains an RLC circuit which corrects the relative amplitude and phase of the various frequency components of the reset-gate signal. The precise values of the components in the RLC circuit are selected according to the attenuation and distortion characteristics of the coaxial cable connecting the drive electronics assembly from the imager assembly.
According to an embodiment of the invention, a cable compensator network which receives a reset-gate signal from a timing generator and provides the reset-gate signal to an imager includes an RLC circuit which receives the reset-gate signal from a buffer circuit and corrects a relative amplitude and phase of a plurality of frequency components of the reset-gate signal; and a low-pass filter which receives the reset-gate signal from the RLC circuit and outputs the reset-gate signal to the imager.
According to an embodiment of the invention, a cable compensator network which receives a reset-gate signal from a timing generator and provides the reset-gate signal to an imager includes correcting means for performing phase and relative amplitude correction of the received reset-gate signal, wherein the correcting means including means for boosting high frequency components of the received reset-gate signal, means for attenuating low frequency components of the received reset-gate signal, and means for limiting attenuation of middle frequency components of the reset-gate signal.
According to an embodiment of the invention, a method for reducing picture noise in a CCD video probe includes the steps of (a) receiving a reset-gate signal from a timing generator; (b) performing phase and relative amplitude correction of the received reset-gate signal; wherein the step of performing includes the steps of boosting high frequency components of the received reset-gate signal, attenuating low frequency components of the received reset-gate signal, and limiting attenuation of middle frequency components of the reset-gate signal; and (c) providing the corrected reset-gate signal to an imager in the CCD video probe.